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Branton Campbell in the lab

Branton J. Campbell

Professor

Department of Physics & Astronomy

Brigham Young University

Provo, UT 84602, USA

Tel: 801-422-5758, Fax: 801-422-0553

Email: branton_campbell[at]byu.edu

//physics.byu.edu/faculty/campbell/

Research Interests

I apply state-of-the-art x-ray and neutron scattering techniques to study local and long-range structures in a variety of complex solids, including fast-ion conductors, ferroelectric relaxors, high-temperature superconductors, and colossal magnetoresistive manganites, where nanoscale structural features influence macroscopic physical properties. This includes the development of symmetry-mode analysis as a tool for the determination, refinement and interpretation of distorted structures involving lattice strains, atomic displacements, magnetic moments and occupational orderings at both commensurate and incommensurate wavevectors.

A Big Table of Space-Group IRs

The irreducible representations (IRs) of the parent symmetry of a system provide a symmetry-motivated parameter set for describing any periodic or aperiodic distortion. The IRs of complete crystallographic space groups and their extensions to (3+d)-dimensional superspace, including all special and non-special k-vectors, commensurate and incommensurate, have now been exhaustively tabulated for the first time. Photo (Wikipedia): Table Mountain, Capetown, South Africa.

Structure, thermal expansion and transport properties

By Li-Ping Li and Branton J. Campbell (et al.)

Abstract: Perovskite oxides BaCe1-xEuxO3-delta (x = 0.05, 0.10, 0.15, and 0.20) were prepared using starting ratios of Ba/(Ce + Eu) = 1.0 and 1.1. X-ray diffraction analysis shows that preparations with a slight initial barium excess crystallized as a single perovskite phase, while the stoichiometric preparations crystallized with a trace of CeO2 due to Ba loss. EPR measurements indicate that the samples have no Ba vacancies and that Eu and Ce ions are in trivalent and tetravalent states, respectively. The single-phase samples synthesized from Ba/(Ce + Eu) = 1.1 preparations are shown to have different cell volume trends versus x, lower average thermal expansion coefficients, and higher proton conductivities in water-containing atmospheres between 80 and 700 degreesC, compared to samples synthesized from the Ba/(Ce + Eu) = 1.0 preparations. These observations are explained in terms of the impurity CeO2) phase, Eu doping-site preferences, and oxygen vacancy generation.

Elucidation of zeolite microstructure by synchrotron X-ray diffuse scattering

By B. J. Campbell (et al.)

Abstract: Single-crystal diffuse scattering measurements can now rapidly probe the three-dimensional structure of subtle defects in microporous framework materials. Diffuse scattering data from natural mordenite crystals are shown to exhibit a complex distribution of weak features which have been mapped out using a synchrotron X-ray source and a CCD detector. Comparison with computer-simulated diffuse scattering patterns yields a detailed three-dimensional columnar defect structure and reveals that roughly one third of the mordenite's columnar defects cooperate to form a block-mosaic pattern of {110} stacking faults.

Enhanced stability of charge and orbital order in La_0.78Sr_2.22Mn₂O₇

By B. J. Campbell (et al.)

Abstract:

Not only is zigzag charge and orbital (CO) order shown to be present in the highly doped (x=0.61) bilayered manganite system La2-2xSr1+2xMn2O7, it demonstrates an enhanced stability relative to the x=0.5 composition where the ordered state might be expected to have the lowest entropy. No magnetic-field dependence is observed up to 5.25 T. The CO ordering temperature also increases from 210 K at x=0.5 to 230 K, and the broad hysteretic conversion of the CO-ordered state to the A-type AF state at low temperatures is only partial. Phase reorganization in this hysteretic mixed-phase region is seen to involve an exponential time relaxation above T=50 K, while reorganization ceases below 50 K, apparently due to the thermally activated nature of the underlying dynamics.

Polaronic orbital polarization in a layered colossal magnetoresistive manganite

By B. J. Campbell (et al.)

Abstract: The striking anisotropy observed in Huang scattering distributions near the intense Bragg reflections of La1.2Sr1.8Mn2O7 is shown to be the result of orbitally polarized polarons in the paramagnetic insulating state above T-C. X-ray single-crystal diffuse scattering patterns from this bilayered colossal magnetoresistive manganite are calculated in terms of the polaronic local structure and compared with experimental measurements. At 300 K, the polaronic e(g) electrons occupy "out-of-plane" (i.e., 3z(2)-r(2)) orbitals, leading to MnO6 octahedra that are Jahn-Teller elongated along the c axis, perpendicular to the perovskite layers. Between 300 K and T-C, however, the "orbital polarization" is shown to shift into the a-b plane (i.e., 3x(2)-r(2) and 3y(2)-r(2) orbitals), allowing the formation of nanoscale polaron correlations above T-C.

Glass transition in the polaron dynamics of colossal magnetoresistive manganites

By B. Campbell (et al.)

Abstract: Neutron scattering measurements on a bilayer manganite near optimal doping show that the short-range polaron correlations are completely dynamic at high T , but then freeze upon cooling to a temperature T*approximate to310 K . This glass transition suggests that the paramagnetic/insulating state arises from an inherent orbital frustration that inhibits the formation of a long-range orbital- and charge-ordered state. Upon further cooling into the ferromagnetic-metallic state (T-C=114 K) , where the polarons melt, the diffuse scattering quickly develops into a propagating, transverse optic phonon.